Residents of the small German town first thought that the impact from the self-detonated bomb was from either a meteor or an earthquake.

At around 4 a.m. this past Sunday, residents of the central German town of Limburg awoke to what they thought was a meteor collision. Indeed, the following day they found a crater measuring 33-feet-wide and deep 13-feet-deep, but local officials were able to confirm that the crater was actually the result of a blast from an unexploded 550-pound bomb dating back to World War II.

The residents weren’t without merit to think that the bomb blast was a meteor, as drone images show, and the explosion itself was big enough to register as a minor tremor of 1.7 on the Richter scale. Fortunately, since the bomb had detonated in the middle of a cornfield nobody was hurt.

Rüdiger Jehn of the European Space Agency was able to determine that the blast was not the work of a meteorite as “A great deal of heat is released during an asteroid impact,” and no heat or melting could be seen from the footage of the crater.

As DW reports, that although authorities initially had not been able to definitively determine cause of the blast, closer inspection yielded “with almost absolute certainty” that the mysterious hole had been the work of a World War II bomb.

Experts estimate that the bomb had weighed 550 pounds and was likely dropped by a fighter plane during the war. The chemical detonator inside the bomb had deteriorated to the point that it triggered the bomb’s fuse and exploded on its own.

Limburg City Spokesman Johannes Laubach said that the discovery was not that surprising, given the area’s geographical history as a former railway depot which made it a prime bombing target during the war. Decades after the end of World War II, the barrage of bombs that covered Europe had been reduced into dormant time bombs. These unexploded bombs can pose a real threat particularly when they are located in highly-populated urban areas.

Philippe Huguen/AFP/Getty ImagesBomb disposal experts lift a British World War II bomb discovered in France.

Just last month, for example, Southwest Londoners were shocked by an unexploded bomb found in the city’s Kingston area. The bomb was discovered near a building site and sparked the evacuation of nearly 1,500 homes within its vicinity.

This year, the discovery of unexploded bombs has been particularly high. There were at least 19 bomb alerts across Germany within the last month alone. Reports of these bombs have come in from the cities of Cologne, Berlin, Hamburg, Stuttgart, as well as from Germany’s countryside. Most alarmingly, some were even found close to the Autobahn station near Nuremberg.

Although uncovering some of these sleeping explosives is expected at some point, the growing number of discoveries by German authorities has likely been exacerbated by the country’s construction boom.

Land excavations for development have often led to the discovery of an unexploded bomb and those discoveries are likely to increase as the number of new construction contracts continue to rise ⁠— in fact, the number of construction contracts in Germany has already grown 11.3 percent since last year.

According to historian Jens Wehner, between 1.3 and 1.4 million bombs were dropped on Germany’s territory during World War II and around 10 percent of these bombs during the war did not explode. However, it is difficult to estimate how many unexploded bombs are left today.

The threat of these bombs may appear alarming, but officials assure that the risk of being hit or affected by one is very rare. They say residents have a higher chance of getting struck by lightning than a self-detonating World War II bomb. These bombs are, however, frequent enough that bomb sweeping has become standard procedure in construction planning in Germany.